Project Summary Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by the formation of extra-skeletal bone known as heterotopic ossification (HO). HO is caused by a series of cellular and tissue- wide events that lead to bone formation within skeletal muscle and other soft connective tissues. All familial and sporadic cases with a classic clinical presentation of FOP carry a gain-of-function heterozygous mutation in ACVR1 (R206H; c.617G>A), a cell surface receptor that mediates bone morphogenetic protein (BMP) signaling, which has been recognized for its chondro- and osteogenic-induction potential. The heterotopic bone that forms in FOP patients is qualitatively normal endochondral bone, however initiation of this bone formation occurs as a result of misdirected cell fate decisions within the affected tissue. HO can form both spontaneously and after injury to the muscle or other soft connective tissue. In addition to ligand-receptor signaling, mechanical cues derived from the physical environment also direct cell fate decisions, with stiff substrates promoting chondrogenic and osteogenic fates and softer substrates promoting neurogenic, adipogenic, and myogenic cell fates. We have observed increased activation of cellular mechanotransduction in progenitor cells harboring the FOP mutation. This proposal seeks to identify the influence of elevated BMP signaling conferred by the Acvr1R206H mutation on cellular mechanical signal transduction (also known as mechanotransduction) and how it affects the ability of muscle stem cells (MuSCs) to regenerate skeletal muscle after injury. I will first determine whether elevated BMP signaling in Acvr1R206H cells interacts with mechanical signaling pathways to alter response to substrate stiffness leading to aberrant cell fate decisions by mesenchymal stem cells (MSCs) (Aim 1). I will also investigate the effect of the Acvr1R206H mutation on the ability of a muscle tissue cell population, muscle stem cells (MuSCs), to repair damaged skeletal muscle tissue (Aim 2). While FOP is a rare genetic disease, HO is a common pathological response to severe tissue trauma scenarios such as blast-initiated injuries and joint- replacement surgeries. Developing a better understanding of HO from a genetic perspective could provide insight into the aberrant mechanisms regulating bone formation and provide targets for pharmaceutical intervention in both genetic and non-genetic causes of HO. Additionally, insights gained from this study will provide us with a better understanding of cellular mechanotransduction and its role in muscle regeneration after injury.